Theory of Mind (ToM) is the ability to predict and understand the mental state of another. While ToM is theorized to play a role in language, we examined whether such a mentalizing ability plays an important role in establishing shared understanding in conversation. Pairs of participants engaged in a Lego model building task in which a director instructed a builder on how to create duplicate models from a prototype that only the director could see. We manipulated whether the director could see (visible condition) or could not see (hidden condition) the builder’s workspace. As predicted, the Mind in the Eyes test (a measure of ToM) predicted accuracy when the workspace was hidden. A high mentalizing ability was an advantage when instructing, resulting in fewer errors, but may be a disadvantage when following instructions. This research indicates that ToM plays a key role in communicating information effectively in conversation.